equivalent of c++ private inheritance with using

[Dan]

What is best way to get equivalent of C++ private inheritance 
combined with using declarations in the drived to expose some 
functionality. For example, suppose I want a basic RateCurve 
class that defers almost entirely to type Array as below. The 
problem is, once RateCurve is moved to a different module, the 
private impl prevents it from working:

Thanks
Dan

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module play.m.x;
import std.stdio;
import std.container;
import std.datetime;

struct DateValue {
    Date date;
    double value;
}

struct RateCurve {
   alias Array!DateValue Impl;
   alias impl this;

   // I am a Array!DateValue, so I can do:
   double getRate(Date d) {
     // access myself to get a rate...
     return 0;
   }

public: // If this is private it is a problem!!! How to fix?
   Impl impl;
}
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import std.stdio;
import std.datetime;
import play.m.x;

void main() {
   static if(0) {
     // Support for following would be great
     auto rc = RateCurve(DateValue(Date(2001,1,1), 0.2),
                         DateValue(Date(2002,1,1), 0.25));
   } else {
     auto rc = RateCurve();
   }
   rc.insert(DateValue(Date(2001,1,1), 0.2));
   // indexing
   writeln("Initial rate is ", rc[0]);
   auto someDate = Date(2001,6,1);
   auto rate = rc.getRate(someDate);
   foreach(r; rc) {
     writeln("on ", r.date, " rate is ", r.value);
   }
}
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